Load average is the average number of processes in the run queue during last X minutes, where X can be 1, 5 or 15. That is, load average is the number of processes which want to run their code on the CPU right now.

High load average can be caused by anything -- apache, exim, spamd, runaway or malicious users processes etc. Though I can't say that 10 is a high load average. If it grows to 20 or 30 and doesn't drop, then there is a reason to look into it further.